Chico is right on the 2" for sound with the X pipe. Plus you are running true duals you don't need two huge diameter pipes going back. GTP's run 2.5 single back so if you think about it 2" double is more than enough.
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04 GTO M6 | HE SS Clutch | Kooks LT's | SLP LM2 | K&N
Soon: APS TT kit
SOLD \/
01 GT | STr5 cam | PT-67 | 18-22psi